A biosensor (or biological sensor) is an analytical device including a biological element and a transducer that converts a biological response into an electrical signal. Certain biosensors involve a selective biochemical reaction between a specific binding material (e.g., an antibody, a receptor, a ligand, etc.) and a target species (e.g., molecule, protein, DNA, virus, bacteria, etc.), and the product of this highly specific reaction is converted into a measurable quantity by a transducer. Other sensors may utilize a non-specific binding material capable of binding multiple types or classes of molecules or other moieties that may be present in a sample, such as may be useful in chemical sensing applications. The term “functionalization material” may be used herein to generally relate to both specific and non-specific binding materials. Transduction methods may be based on various principles, such as electrochemical, optical, electrical, acoustic, and so on. Among these, acoustic transduction offers a number of potential advantages, such as being real time, label-free, and low cost, as well as exhibiting high sensitivity.
An acoustic wave device employs an acoustic wave that propagates through or on the surface of a piezoelectric material, whereby any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Presence of functionalization material embodied in a specific binding material along an active region of an acoustic wave device permits a specific analyte to be bound to the specific binding material, thereby altering the mass being vibrated by the acoustic wave and altering the wave propagation characteristics (e.g., velocity, thereby altering resonance frequency). Changes in velocity can be monitored by measuring the frequency, amplitude-magnitude, or phase characteristics of the acoustic wave device, and can be correlated to a physical quantity being measured.
In the case of a piezoelectric crystal resonator, an acoustic wave may embody either a bulk acoustic wave (BAW) propagating through the interior of a piezoelectric material, or a surface acoustic wave (SAW) propagating on the surface of the piezoelectric material. SAW devices involve transduction of acoustic waves (commonly including two-dimensional Rayleigh waves) utilizing interdigital transducers along the surface of a piezoelectric material, with the waves being confined to a penetration depth of about one wavelength. BAW devices typically involve transduction of an acoustic wave using electrodes arranged on opposing top and bottom surfaces of a piezoelectric material. In a BAW device, three wave modes can propagate, namely, one longitudinal mode (embodying longitudinal waves, also called compressional/extensional waves), and two shear modes (embodying shear waves, also called transverse waves), with longitudinal and shear modes respectively identifying vibrations where particle motion is parallel to or perpendicular to the direction of wave propagation. The longitudinal mode is characterized by compression and elongation in the direction of the propagation, whereas the shear modes consist of motion perpendicular to the direction of propagation with no local change of volume. Longitudinal and shear modes propagate at different velocities. In practice, these modes are not necessarily pure modes, as the particle vibration, or polarization, is neither purely parallel nor purely perpendicular to the propagation direction. The propagation characteristics of the respective modes depend on the material properties and propagation direction respective to the c-axis orientations. The ability to create shear displacements is beneficial for operation of acoustic wave devices with fluids (e.g., liquids) because shear waves do not impart significant energy into fluids.
Certain piezoelectric thin films are capable of exciting both longitudinal and shear mode resonance, such as hexagonal crystal structure piezoelectric materials including (but not limited to) aluminum nitride (AlN) and zinc oxide (ZnO). To excite a wave including a shear mode using a piezoelectric material arranged between electrodes, a polarization axis in a piezoelectric thin film must generally be non-perpendicular to (e.g., tilted relative to) the film plane. In biological sensing applications involving liquid media, the shear component of the resonator is used. In such applications, piezoelectric material may be grown with a c-axis orientation distribution that is non-perpendicular relative to a face of an underlying substrate to enable a BAW resonator structure to exhibit a dominant shear response upon application of an alternating current signal across electrodes thereof.
Typically, BAW devices are fabricated by micro-electro-mechanical systems (MEMS) fabrication techniques owing to the need to provide microscale features suitable for facilitating high frequency operation. In the context of biosensors, functionalization materials (e.g., specific binding materials; also known as bioactive probes or agents) may be deposited on sensor surfaces by microarray spotting (also known as microarray printing) using a microarray spotting needle. Functionalization materials providing non-specific binding utility (e.g., permitting binding of multiple types or species of molecules) may also be used in certain contexts, such as chemical sensing.
Sensing devices incorporating BAW resonator structures and intended for use with fluids may define fluidic passages configured to direct fluid over an active region. The small dimensions associated with fluidic passages of these devices may make such passages susceptible to occlusion or blockage, such as with particulate material, cells, and/or bubbles (which may be collectively referred to herein as “obstruction media”), wherein complete blockage of a fluidic passage (e.g., a channel, a chamber, or the like) may render a fluidic device inoperable. Additionally, it may be challenging to detect certain types of analytes that are present in low concentration and/or exhibit low rates of binding to functionalization material.
Accordingly, there is a need for devices incorporating bulk acoustic wave resonator structures suitable for operation in the presence of liquid for biosensing or biochemical sensing applications that overcome limitations associated with conventional devices.